System and method for adaptive brake application and initial skid detection

ABSTRACT

The adaptive brake application and initial skid detection system allows rapid brake application and prevents deep initial skids. Brake torque is compared with a predetermined threshold brake torque. Wheel velocity error signals are also generated to indicated the difference between the wheel velocity and a reference velocity signal. A torque bias modulator integrator responsive to brake torque signals adjusts the wheel velocity error signals to provide an anti-skid control signal. The torque bias modulator integrator can also be initialized to the value of the measured brake torque when the wheel velocity error signals indicate the beginning of a skid. Brake torque difference signals are generated to indicate the difference between brake torque and a commanded brake torque, and an adjusted brake torque error signal is generated in response to the brake torque difference signals.

RELATED APPLICATIONS

This is a continuation of Ser. No. 10/803,210, filed Mar. 17, 2004, nowU.S. Pat. No. 6,916,075, which is a continuation of Ser. No. 10/225,092,filed Aug. 21, 2002, now U.S. Pat. No. 6,722,745, which is acontinuation-in-part of Ser. No. 10/027,820, filed Dec. 21, 2001, nowU.S. Pat. No. 6,527,350, which is a continuation of Ser. No. 09/887,581,filed Jun. 21, 2001, now U.S. Pat. No. 6,345,872, which is a divisionalof Ser. No. 09/591,093, filed Jun. 8, 2000, now U.S. Pat. No. 6,299,262,which is a continuation of Ser. No. 08/850,680, filed May 2, 1997, nowU.S. Pat. No. 6,132,016.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to deceleration control systems forvehicles, and more particularly concerns an adaptive brake applicationand initial skid detection system for braking of one or more wheels ofan aircraft during landing that prevents deep initial skids, and to thusallow rapid brake application in a controlled manner.

2. Description of the Related Art

Anti-skid and automatic braking systems commonly have been provided oncommercial and large turbine aircraft to aid the deceleration of theaircraft upon landing. Modern anti-skid systems typically optimizebraking efficiency by adapting to runway conditions and other factorsaffecting braking to maximize deceleration, corresponding to the levelof brake pressure selected by the pilot. In conventional antiskidsystems, brakes are typically applied mechanically via a metering valveby the pilot, and as soon as the wheel brake pressure approaches theskid level, such as when an initial skid is detected, a brake pressurevalue is used to initialize the antiskid control system. However, it hasbeen found that the success of this method can be affected by suchfactors as the mode of aircraft operation, aircraft weight, tire/runwayinterfaces, and the like. It would therefore be desirable to provide anadaptive brake application system that can adjust brake pressure ortorque application to account for such factors.

Furthermore, rapid pedal application by an aircraft pilot also can oftencreate deep initial skids before an effective antiskidding brakepressure or brake torque is determined and skidding is effectivelycontrolled by conventional antiskid and brake control systems.Eliminating or reducing initial skids would result in shorter aircraftstopping distances, which allow the aircraft to land on shorter runways,and can result in reduced tire wear. It would thus be desirable toprovide an initial skid detection system to automatically anticipateinitial skid conditions and adjust to prevent deep initial skids, toallow the pilot to depress the brake pedals at any rate, while stillproviding for rapid brake application in a controlled manner. Thepresent invention provides an adaptive brake application and initialskid detection system that meets these needs.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides for anadaptive brake application and initial skid detection system that allowsrapid brake application, while preventing deep initial skids, byimplementation of a skid anticipation system that is initialized as soonas a wheel approaches a skid level to reduce brake application pressureor torque and to apply brakes in a controlled manner.

The invention accordingly provides for a “smart” brake application andinitial skid detection system for braking of a wheel of an aircraftduring landing. The system is applicable to one or more wheels having awheel brake for applying brake torque to the wheel. A brake torquesensor generates brake torque signals that are a function of the brakingtorque applied to the wheel brake, and the brake torque signals arecompared with a predetermined threshold brake torque. A wheel speedtransducer produces wheel speed signals that are a function of therotational speed of the wheel, and a wheel velocity signal is generatedbased upon the wheel speed signals. The wheel velocity is compared witha reference velocity signal for generating wheel velocity error signalsindicative of the difference between the aircraft wheel velocity signalsand the reference velocity signal. A torque bias modulator integrator isalso provided that is responsive to brake torque signals for adjustingthe wheel velocity error signals to provide an anti-skid control signal,and in one currently preferred embodiment the torque bias modulatorintegrator is initialized with the predetermined threshold brake torqueplus a predetermined constant torque value. A command processorgenerates a command brake torque signal generated in response to adeceleration command, and brake torque comparison means are provided forcomparing the brake torque signals with the command brake torque signalfor generating brake torque difference signals indicative of thedifference between the brake torque signals and the command brake torquesignal. Control means provide an adjusted brake torque signal to thewheel brake to control the wheel brake independently of operator brakeapplication, in response to the brake torque difference signals. Inanother presently preferred embodiment, the torque bias modulatorintegrator is initialized to the value of a measured brake torque whenthe wheel velocity error signal indicates the beginning of a skid.

In a currently preferred embodiment, means are also provided foradjusting the brake torque error signals by a proportional torque gain,an integral torque gain, and a differential torque gain. In anotherpresently preferred embodiment, transient control means for providing aproportional control signal and compensation network means, bothresponsive to the velocity error signal, are also provided, and theoutputs of the transient control means and compensation network meansare summed with the output of the torque bias modulator integrator.

From the above, it can be seen that the present invention provides asystem and method to initiate brake control after rapid application ofthe brake pedal, but before the onset of skidding occurs. These andother aspects and advantages of the invention will become apparent fromthe following detailed description and the accompanying drawings, whichillustrate by way of example the features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a “smart” brake application and initialskid detection system for an aircraft, according to the principles ofthe invention;

FIG. 2 shows two charts relating brake pressure, wheel velocity andbrake torque over time for the “smart” brake application and initialskid detection system of the invention; and

FIG. 3 is a chart illustrating the brake force to brake slip curve forthe “smart” brake application and initial skid detection system of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Effectiveness of conventional antiskid systems can be affected by themode of aircraft operation, aircraft weight, tire/runway interfaces, andsimilar factors. Rapid aircraft brake pedal application, especiallypanic application, can also create deep initial skids before antiskidcontrol is initiated, resulting in lengthening of aircraft stoppingdistances and increased tire wear.

The present invention provides a system and method for direction of theconditions for the onset of skidding, and the initiation of brakecontrol to prevent deep skids prior to controlled deceleration. Withreference to FIG. 1, the invention is embodied in an adaptive, or“smart” brake application and initial skid detection system 10 which canbe used in aircraft braking systems, and as installed for an aircraftpreferably includes a wheel speed transducer 12 for each wheel brake 14of a wheel 15 of the aircraft, for measuring wheel speed and generatingwheel speed signals that are a function of the rotational speed of thebrake wheel. The wheel speed signal is typically converted to a signalrepresenting the velocity of the aircraft by a velocity converter 16,and compared with a desired reference velocity in velocity comparator18, to generate wheel velocity error signals indicative of thedifference between the wheel velocity signals from each braked wheel andthe reference velocity signal. The output of the velocity comparator isreferred to as slip velocity (Vs) or velocity error. The velocity errorsignals are adjusted by a torque bias modulator control means (TBM)integrator 20, the transient control means 22, and compensation network24, the outputs of which are summed at summing junction 26 to provide ananti-skid control signal 28 received by the command processor 30,typically a microprocessor. The TBM integrator in the antiskid loopdictates the maximum allowable control torque level during braking. TheTBM integrator is typically slower in response than other controlparameters needed to detect and control initial skid. When no skid isdetected, this integrator allows full system torque to the brakes.

The position of the aircraft brake pedal 32 operated by the pilot istypically read by a microcontroller 33 that generates a brake pedalcommand signal 34, from which a torque application profile isdetermined. The command processor 30 receives the brake pedal commandsignal, the anti-skid control signal 28 via feedback line 36, andpreferably also receives a locked wheel protection signal 38 indicatingwhether a wheel is locked, and a touchdown/hydroplaning protectionsignal 40, to guard against hydroplaning of a wheel on touchdown at highspeeds. In a currently preferred embodiment, the command processoroperates on the lowest input of the locked wheel protection signal, thetouchdown protection signal, the pedal signal, and the antiskid signal.The commanded brake torque signal output 42 of the command processor iscompared with the brake torque feedback signal 44 from brake torquesensor 46 by comparator 48, which generates an output torque errorsignal 50.

In a currently preferred embodiment, the brake torque error signals arealso adjusted by a proportional gain by proportional gain circuitry 52,an integral gain by integral gain circuitry 54, and a differential gainby differential gain circuitry 55 that together form a PID control loop,and the outputs of which are summed at summing junction 56 to provide anadjusted brake torque signal 57. The adjusted brake torque signal isalso typically amplified by valve amplifier 58 to provide an amplifiedbrake control signal applied to the brake control valve 60 that controlsthe application of pressurized brake fluid from system torque 62 to thewheel brake.

In a presently preferred embodiment, the functions of the elements inthe block 63 are performed by one or more microprocessors underappropriate software control, although alternatively these or analogousfunctions may be performed by suitable hardware components. It will beappreciated by those skilled in the art that the component parametersand configurations will vary from aircraft to aircraft and that there isthus wide variability in how the system can be used.

“Smart” Brake Application:

Referring to FIG. 2, brake application is allowed without any ratelimiting until brake torque is detected at a preset value 64, typicallynear the brake contact torque 66, at which point the brake torquecommences to rise. Then the TBM integrator is initialized by a means forintitalizing the torque bias modulator integrator, such as the TORQUEINIT unit 21 shown in FIG. 1, to the preset brake torque value plus apredetermined constant increment of torque, at 68, which corresponds tothe peak of the brake torque-slip curve 70 shown in FIGS. 2 and 3. Theoutput of the TBM integrator is shown as dotted line 72, and thecommanded brake torque output is shown as line 74. The wheel velocity isshown as line 76, and brake torque is shown as line 78. As isillustrated in FIG. 2, the initialization of the TBM integrator forcesthe TBM integrator to track the brake application profile beginning at69, thus preventing any substantial overshoot.

“Smart” Skid Detection:

When a wheel approaches the skid level, such as when Vs is detected tobe greater than the preset wheel velocity limit, then the TBM integratoris initialized with the value of brake torque feedback at the time thatVs is greater than the preset limit. This method ensures correctinitialization of the TBM integrator. The brake torque at the time of aninitial skid is what the TBM integrator needs to be for the immediatecontrol without multiple initial skids. Therefore a fast response of theTBM integrator is insured to an otherwise slow moving control function.

It should be apparent that this invention is not limited to velocityerror type systems, and that the invention is also applicable to otherbrake control skid detection concepts, such as rate control/detection,as well as any system that monitors the brake application and pressureor torque.

From the above, it will be recognized by those skilled in the art thatthe present invention provides a new and novel method and apparatus toindicate brake control prior to the initialization of skids and toprevent overshoot and instability after brake control is begun.

It will also be apparent from the foregoing that while particular formsof the invention have been illustrated and described, variousmodifications can be made without departing from the spirit and scope ofthe invention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

1. In an adaptive brake application and initial skid detection systemfor braking of a wheel of a vehicle, the vehicle having a braking systemincluding a wheel brake for braking the wheel, means for generating awheel velocity signal indicative of wheel velocity, velocity referencegenerating means for generating a reference velocity signal, and wheelvelocity comparison means for comparing said wheel velocity signal withsaid reference velocity signal for generating wheel velocity errorsignals indicative of the difference between said wheel velocity signaland said reference velocity signal for controlling braking the wheel,the improvement comprising: a brake torque sensor for generating braketorque signals that are a function of the brake torque applied to thewheel; means for comparing said brake torque signals with apredetermined threshold brake torque; a torque bias modulator integratorresponsive to said wheel velocity signals for adjusting said wheelvelocity error signals to provide an anti-skid control signal; commandbrake torque signal generating means for generating a command braketorque signal in response to a deceleration command; brake torquecomparison means for comparing said brake torque signals with saidcommand brake torque signal for generating brake torque differencesignals indicative of the difference between said brake torque signalsand said command brake torque signal; control means for providing anadjusted brake torque signal to said wheel brake to control said wheelbrake independently of operator brake application, in response to saidbrake torque difference signals; and means for initializing the torquebias modulator integrator with a predetermined threshold brake torqueplus a predetermined constant torque value.